The Clark Tartan was 'taken' from the Clergy Tartan (also called "Priest") ;
'Clark' 'Clerk' comes from "Cleric" (Clergy).
The Clark clan 'comandeered' the Tartan which was brightly colored, the Protestant Calvinist/ Presbyterians felt the Clergy should have less frivolity so they began to mute the colors for Clergy, while the Clarks kept the brighter tartan.
"Clergy" is used by mpst Denominations, but some Catholics sometimes tend to maintain the original (older) brighter version - and are often confused with the Clark family, or the Clerks get mistaken for Clergy in Catholic Circles.
The Highlanders remained Catholic, while the Lowladers embraced John Knox's Presbyterianism, but both clergy used the same tartan nevertheless.